1. Field of Use
This invention relates to gated filament structures for a field emission display with filaments positioned in apertures. The relative position of the majority of each filament tip to its associated aperture is substantially the same for a majority of the filament tips of the display. This relationship is maintained even for large displays where there are nonuniformities in the thickness of the insulating layer or in the plating of the filaments.
2. Description of the Related Art
Field emission displays include a faceplate, a backplate and connecting walls around the periphery of the faceplate and backplate, forming a sealed vacuum envelope. In some field emission displays, the envelope is held at vacuum pressure, which can be about 1xc3x9710xe2x88x927 torr or less. The interior surface of the faceplate is coated with light emissive elements, such as phosphor or phosphor patterns, which define an active region of the display. Field emission cathodes, such as cones and filaments, are located adjacent to the backplate. Application of an appropriate voltage at the extraction electrode releases electrons which are accelerated toward the phosphors on the faceplate. The accelerated electrons strike their targeted phosphors, causing the phosphors to emit light seen by the viewer at the exterior of the faceplate. Emitted electrons for each of the sets of emitters are intended to strike only certain targeted phosphors.
A variety of methods for forming field emitters are known.
U.S. Pat. No. 3,655,241 discloses fabricating field emitters using a screen with arrays of circular or square openings that is placed above a substrate electrode. A deposition is performed simultaneously from two sources. One of the sources consists of an emitter-forming metal, such as molybdenum, and atoms are deposited in a direction perpendicular to the substrate electrode. The other source consists of a closure material, such as a molybdenum-alumina composite. Atoms of the closure material are caused to impinge on the screen at a small angle to the substrate. The closure material progressively closes the openings in the screen. Thus the emitter-forming metal is deposited in the shape of cones or pyramids, depending on whether the screen openings are circular or square.
Another method of creating field emitters is disclosed in U.S. Pat. No. 5,164,632. Part of an aluminum plate is anodically oxidized to create a thin alumina layer having pores that extend nearly all the way through the alumina. An electrolytic technique is used to fill the pores with gold for the field emitters. An address line is formed over the filled pores along the alumina side of the structure, after which the remaining aluminum and part of the adjoining alumina are removed along the opposite side of the structure to re-expose the gold in the pores. Part of the re-exposed gold is removed during an ion-milling process utilized to sharpen the field emitters. Gold is then evaporatively deposited onto the alumina and partly into the pores to form the gate electrode.
Field emitters are fabricated in U.S. Pat. No. 5,150,192 by creating openings partway through a substrate by etching through a mask formed on the bottom of the substrate. Metal is deposited along the walls of the openings and along the lower substrate surface. A portion of the thickness of the substrate is removed along the upper surface. A gate electrode is then formed by a deposition/planarization procedure. Cavities are provided along the upper substrate surface after which the hollow metal portions in the openings are sharpened to complete the field emitter structures.
However, large area field emission displays require a relatively strong substrate for supporting the field emitters extending across the large emitter area. The requisite substrate thickness is typically several hundred microns to 10 mm or more.
The fabrication methods in U.S. Pat. Nos. 5,164,632 and 5,150,192 make it very difficult to attach the field emitters to the substrates of thickness required for large area displays.
In U.S. Pat. No. 4,940,916, a gated area field emitter consists of cones formed on a highly resistive layer that overlies a highly conductive layer situated on an electrically insulating supporting structure. For a thickness of 0.1 to 1 microns, the highly resistive layer has a resistivity of 104 to 105 ohm-cm. The resistive layer limits the currents through the electron-emissive cones so as to protect the field emitter from breakdown and short circuits.
It is desirable to have uniformity of emission from the cathodes. A field emission cathode relies on there being a very strong electric field at the surface of a filament or generally on the surface of the cathode. Creation of the strong field is dependent on, (i) the sharpness of the cathode tip and (ii) the proximity of the extraction electrode (gate) and the cathode. Application of the voltage between these two electrodes produces the strong electric field. Emission nonuniformity is related to the nonuniformity in the relative positions of the emitter tip and the gate. Emission nonuniformity can also result from differences in the sharpness of the emitting tips.
Busta, xe2x80x9cVacuum Microelectronics-1992,xe2x80x9d J. Micromech. Microeng., Vol. 2, 1992 pp. 43-74 provides a general review of field-emission devices. Among other things, Busta discusses Utsumi, xe2x80x9cKeynote Address, Vacuum Microelectronics: What""s New and Exciting,xe2x80x9d IEEE Trans. Elect. Dev., October 1990, pp. 2276-2283, who suggests that a filament with a rounded end is the best shape for a field emitter. Also of interest is Fischer et al., xe2x80x9cProduction and Use of Nuclear Tracks: Imprinting Structure on Solids,xe2x80x9d Rev. Mod. Phys., October 1983, pp. 907-948, which deals with the use of charged-particle tracks in manufacturing field emitters according to a replica technique.
A well collimated source of evaporant, as taught in U.S. Pat. No. 3,655,241, is necessary in order to obtain uniformity of cone or filament formation across the entire field emission display. In order to maintain a collimated source, the majority of evaporant is deposited on interior surfaces of the evaporation equipment. The combination of the expensive of the evaporation equipment, and the wastage of evaporant, is undesirable for commercial manufacturing and is compounded as the size of the display increases. With large displays, there are nonuniformities in the thickness of the insulating layer and the plating of the filaments.
It would be desirable to provide a gated filament structure for a field emission display where each filament and filament tip is positioned in a gate aperture. It would further be desirable to provide a large field emission display in which the relative positions of the filament tips to their associated apertures are substantially the same for a majority of the filament tips of the display. There is a need to maintain this relationship for large displays which more nonuniformities in the thickness of the insulating layer and in the plating of the filaments.
Accordingly, it is an object of the invention to provide gated filament structures for large field emission displays.
Another object of the invention is to provide gated filament structures for large field emission displays that have nonuniformities in the thickness of the insulating layer or nonuniformity of plating of the filaments.
A further object of the invention is to provide gated filament structures for a large field emission display where the gate is used to define the position of the filament tip.
Still another object of the invention is to provide gated filament structures for a large field emission display where the gate is used to define the geometry of the filament tip.
Yet another object of the invention is to provide gated filament structures which have sharpened filament tip geometries.
Another object of the invention is to provide gated filament structures which have sharpened filament tip geometries that are positioned between a top planar surface and a bottom planar surface of the gate.
A further object of the invention is to provide gated filament structures that are electroplated.
Another object of the invention is to provide gated filament structures for a large field emission display that are vertically self aligned in its associated aperture.
These and other objects of the invention are achieved in a gated filament structure for a field emission display that includes a plurality off filaments. A gated filament structure for a field emission display includes a plurality of filaments. Included is a substrate, an insulating layer positioned adjacent to the substrate, and a metal gate layer, including a plurality of gates positioned adjacent to the insulating layer. The metal gate layer has an average thickness xe2x80x9csxe2x80x9d and a top metal gate layer planar surface that is substantially parallel to a bottom metal gate layer planar surface. A plurality of apertures extending through each gate formed in the metal gate layer. Each aperture has an average width xe2x80x9crxe2x80x9d along a bottom planar surface of the aperture. Each aperture defines a midpoint plane positioned parallel to and equally distant from the top metal gate layer planar surface and the bottom metal gate layer planar surface. A plurality of gated filaments are individually positioned in an aperture. Each filament has a filament axis. The intersection of the filament axis and the midpoint plane defines a point xe2x80x9cOxe2x80x9d. Each filament includes a filament tip terminating at a point xe2x80x9cAxe2x80x9d. A majority of all filapement tips of the display have a length xe2x80x9cLxe2x80x9d between each filament tip at point A and point O along the filament axis where,
Lxe2x89xa6(s+r)/2.
It is preferred that at least 75% of all filament tips of the display have this relationship for points A and O, more preferably at least 90% of the filament tips have this relationship.
The majority of filament tips can, (i) extend between the top and bottom metal gate layer surfaces, (ii) extend below the bottom metal gate layer surface, or (iii) extend above the top metal gate layer surface.
Each filament of the display can be electroplated.
In another embodiment, the gated filament structure for a field emission device includes a substrate.
Additionally, the majority of the filament tips can extend beyond the top metal gate layer planar surface, or below the bottom metal gate layer planar surface.
Further, each filament can be electroplated. Each filament is vertically self aligned in its associated aperture.